Corrosion resistant alloy



atente .Fuly 29, 1941 TED STATES FATE OF F E CORROSION RESISTANT I Peter l'ayson, New York, N. 1., or to Crucible Steel Company of America, New York, N. Y.',

a corporation of New Jersey No Drawing. Application July 28, 1940, Serial No. 347,173

This invention pertains to corrosion resisting ferrous alloys and articles thereof, and its principal object is to provide a relatively economical and workable alloy characterized by high resistance to hot sulfuric acid and hence especially 5 adapted for use in metallurgical pickling tanks,

dye vats, etc.

One of the most widely used processes in th metallurgical industry is pickling. Not only is the'flnal product pickled either to give a uniform 1o appearance or as a preparation for an ornamental or protective finish such as painting. plating, etc., but the metal or alloy must be pickled many times during the course 0! fabrication, i. e. be-

fore cold rolling, after annealing, heat treatment, 15

etc. Of the various solutions used in the iron and steel industry for pickling, perhaps the most common are suluiuric acid solutions. A very common pickle for steels is a to sulfuric acid solution at about 150 to 160 F. Naturally,

such solutions are highly corrosive and readily attack not on1y carbon and low alloy steels, but also the stainless steels now available. Therefore, it has been necessary to construct the pickling tanks in a most complicated and expensive manner to provide both strength and corrosion resistance. For example, the outside has been made of steel sheet to give the strength required to withstand the weight of the pickling solution and of the-material being pickled. This outer 0 steel tank is then lined with rubberto resist the acid pickling solution and brick sheathed using an acid proof cement. Another type of tank irequentlyusedis a woodentankbmltnpinlayers with an intermediate asphalt lining. The tie rods have been made 'iro'ni corrosionresisting alloys, such as the following:

7 Although this alloy has reasonably good reshtance to sulfuric acid, its blah costs of fabrication have prevented any appreciable use of it for the tankitself.- Noneoithesetanlm'hasprovenvery satisfactory.

large losses of expensive pickling acid. through leaksinthetankas well astobighce costs. There are, naturally, other types of tanks The combined action of tempera- 5o 7 'ture and acid rapidly rats the 'wood leading to perhaps the most widely used in the iron and steel industry and certainly are representative of the complexity of the requirements to be met by pickling tanks. i

The severity of these requirements and the tremendous field of application has encouraged much tedius research in. an attempt to develop a ierrous alloy that would be suitable for the construction of pickling tanks without non- Despite the time, effort and metallic linings. expense, nothing prior to the present invention has been found that will simultaneously: resist the usual pickling solutions of 10-15% sulfuric acid at -160 F4 resist such pickling solutions contaminated with iron salts after long time use; resist these sulfuric acid solutions to which a small amount of salt had been added for pickling corrosion resist-in: steels; have an adequate margin oi saiety to resist inadvertent increases in concentration or temperature of the pickling solution, such as often may occur in commercial operation; be strong enough to support the weight of the solution and oi the articles bein pickled; be commercially and easily fabricatednot only-into rounds such as may be used for tie rods, but also into thin sheet for the tank itself; be readilywelded and not necessarily require heat treatment after welding; and rbl in price. v

In accordance with this invention, a torrent:

alloy is provided which meets all oi these requirements and which has, in its broader aspects, the following p I Percent Carbom About ,0to 0.25 Manganese About 0 to 2.00

' Silicon. About 0to 1.25

Nlckel About 10.0 to 16.0- Chromium About 20.0to28.0 Molybdenum About 0.5a 5.0 Copper About 0.5to 5.0

wherein the combined content of nickel and chromium is about 33% or more. That is to say, the minimum content for nickel and'ohromium together should be about 33%. For reasons ex- 'plained herelnaiter, a preferred and more restricted analysis is the following:

Carbon About 0to 0.10 Manganese"... About 0.8to 1.0 Sillcon About 0.31:0 0.75 Nickel About 11.0 to 14.0 Chromium About 23.0to28.0 Molybdenum About 1.0 to 1.5

in use today but the above mentioned types are 55 Copper.-.--.. About 1.0 to 1.5

wherein, as before, the combined content of chromium and nickel is about 33% or more.

The fact, as shown by the test results presented below, that the above alloy containing little or no silicon, relatively low nickel, and appreciable manganese, is highly resistant to attack by hot sulfuric acid, is quite surprising in view of the prior teaching on this subject which is to the effect that high nickel, up to 40%, and high silicon, up to 15%, impart increased resistance to such attack, while manganese is detrimental in all proportions. The general acceptance of such teaching heretofore is shown by the fact that all commercial ferrous alloys claimed to have resistance to hot sulfuric acid, contain high silicon, high nickel and low manganese, such as the prior alloy above referred to containing 23 to 25% nickel, 2.75 to 3.75% silicon and manganese under 0.5%. Consequently the composition of the alloy comprising the present invention is diametrically opposed to prior teachings. Nevertheless, the corrosion resistance of the new alloy is superior to the old as shown by the comparative tests given in Table II below based on the alloys'A in accordance with the prior art and B in accordance with the present invention, given in Table I:

Table I Not only does Alloy B in accordance with the present invention have a much lower rate of corrosion in spite of its lower alloy content, but also much greater ease of fabrication than Alloy A, a typical commercial alloy supposed to be resistant to sulfuric acid. I

The carbon content of the new alloy should not exceed 0.25%, as higher carbon would make welding less easy and would make heat treatment after welding necessary. This latter requirement would obviously be difllcult in the case of large pickling tanks.

Manganese, as is well known, improves the hot workability of austenitic alloys, and can be present in considerable amounts without deleteriouslyafiecting the corrosion resistance of the present alloy.

As mentioned above, high silicon has hitherto been considered necessary in steels to be exposed to corrosion by hot sulfuric acid solutions. However, the balance of the composition of the present alloy is such that excellent resistance to hot sulfuric acid is obtained with low silicon. Moreover, this low silicon content. is extremely beneficial in fabrication, especially in cold rollmg. 1.25% silicon is about the maximum amount etc., such as are required for tanks; generally, a somewhat lower content, 0.3 to 0.75%, is preferable. I

Table III Analysis, percent Average corrosion rate in 10% Alloy gh oi t 150 n es per 0 Ni O! Mo Cu year 0. 07 11. 85 22. 44 2. l2 0. 07 ll. 70 23. 35 l. 24 1. 00 0.07 11.79 23.23 2.21 1.24 0. (B 11.90 23. 18 1.30 0.093 0. 07 12. 11 23. 54 2.34 0. 005 0. ne 11. 70 23. 43 1. 28 1. 19 0. 004 0. 07 ll. 86 23. 47 l. 32 2.13 0. 002 0. 09 ll. 49 23. 81 2. 42 1. 21 0. 001 0. 07 ll. 27 21. 94 2. 14 2. 26 0. 001

While, as shown above, additions of molybdenum and copper increase the corrosion resistance, this eifect may be even further enhanced by the simultaneous presence of both of these elements. The results also show that for imparting a given degree of resistance, the more expensive molybdenum may be replaced in part by the less expensive copper, as shown for example by a comparison of the results for steels G and H. The addition of copper along with molybdenum is also desirable from the standpoint of workability of the alloy, because molybdenum, owing to its ferrite-forming tendencies, increases the d101- culty of hot rolling, which effect is oilset by the austenite forming tendencies of the copper additions.

In the new alloy which, as shown above, contains about 10 to 18% nickel and about 20 to 28% chromium, the sum of the nickel plus chromium content should be about 33% or more. In other words, if the chromium content is on the low side of the range, the nickel content should be on the high side, and vice versa. The importance of this ratio is shown in the following test results in Table V based on the analyses of Table IV.

Table IV Analysis, percent Alloy 0 Ni Cr Mo Cu 0. 10 9. 34 19.38 1.49 1. 22 0.1 8.87 21.22 2.25 1.97 OJB 9. 18 19.95 2. 26 1. 19 0. 07 11.27 21.94 2. 14 2. 25

Table 1! Average eorro- Percent lion rate in Alloy Cr 1m 10% H.804 at i 1; inches per year 28. 72 0. 2% 29. (II 0. W1 20. 18 0. 210 33. 21 0. 000 35. 13 0. (I!) 85. 0. one 7 $5.33 0.000

Obviously, the alloy described in the present invention can be used in other applications involvin: resistance to hot sulturic um solutions, for 2. A corrosion resisting alloy characterized by example, in the dyeing industry, etc etc. high resistance to hot sulfuric acid, containing: I claim: about 11 to 14% nickel and about 23 to 26% l. A corrosion resisting alloy charaoterizedby chromium, with a minimum content for nickel hish resistance to hot sulfuric acid. containing: 5 and chromium together 01 about 33%, up to about 20 to 28% chromium and about 10 to 16% about 0.1% carbon, about 0.8 to 1.6% manganese. nickel, with a minimum content for nickel and about 0.3 to 0.75% si1icon, about 1 to 1.5% molybchromium together of about 33%, up to about denum, about 1 to 1.5% copper, and the balance 0.25% carbon, up to about 2% manganese, up to. substantially all iron. about l.25% silicon, about 0.5 to 5% molybdenum, PETER PAYSON. about 0.5 to 5% copper, and the balance substantially all iron. 

